The present invention relates to an electro-photographic device, and, in particular, it relates to a laser power regulating means for laser control and a method therefor.
With regard to laser power setting for an electrophotographic device, control of the amount of laser beam emission is performed by making use of a monitor current Im which is generated at a monitor pin photo diode provided in a laser diode. More specifically, after converting the monitor current into a voltage, the output thereof is controlled by feeding back the same to a regulation circuit in a laser power output control means. In the regulation circuit, the current which is to flow into the laser diode itself is controlled so as to keep the fed back voltage at a predetermined value. In order to convert the monitor current to a voltage a resistor is added; however, because of the variability in the monitor current characteristic of a laser diode, a fine adjustment is generally performed by making use of a variable resistor.
Further, in order to control the laser power variably, a reference signal is fetched and the gradation of the laser power is set. Depending on the condition and environment of the electrophotographic device, the laser power is varied, to thereby stabilize the image quality thereof.
In the laser beam emission control means provided with the above-described conventional gradation, the regulation center of the laser power is positioned at the center of the gradation, and the regulation is performed by the added variable resistor for converting the monitor current to a voltage, so as to obtain a predetermined output value. With regard to the gradation characteristic of laser power regulated using such a method, the laser power variability between electro-photographic devices increases as the regulation moves away from the center of the gradation. Because there is a variability in the characteristic between monitor current and slope efficiency (variation rate of laser beam emission amount with respect to the current flowing through a laser diode) of the laser diode, the slope of the laser power gradation is determined depending on the above characteristic. As a result, there arises a problem in that the tint of the final output images differs between electrophotographic devices.
An object of the present invention is to stabilize the laser power output value at any stage of the gradation and to reduce the variability in image quality between electrophotographic devices.
In order to resolve the above-described conventional problems, the present invention proposes a method in which. laser power is regulated at two predetermined points of the gradation. More specifically, in accordance with the present invention, a first laser power regulating means, which regulates the magnitude of laser power output, and a second laser power regulating means which regulates the slope of laser power gradation characteristic, are provided.